The present invention relates to a whole-body magnetic resonance imaging apparatus and more specifically to a magnetic resonance imaging apparatus which permits a human subject under examination to have a sensation of openness or easiness when placed within the gantry.
This application is based on Japanese Patent Application No. 10-275351, filed Sep. 29, 1998, the entire content of which is incorporated herein by reference.
In general, in a whole-body magnetic resonance imaging apparatus, a static field magnet and gradient field coil are placed within a gantry (apparatus""s cover). The static field magnet consists of either a superconducting magnet, a non-superconducting magnet, or a permanent magnet. The gradient field coil is composed of three coils which are usually referred to as Gx, Gy and Gz coils and produce gradient magnetic fields which linearly vary in intensity in the orthogonal x-, y-, and z-axis directions. Normally, the z-axis direction is coincident with the direction of body axis of a human subject laid down on the couch within the gantry, the x-axis direction is coincident with the direction of width of the human subject, and the y-axis direction is coincident with the direction of thickness of the human subject.
The gradient magnetic fields are used to apply linear gradients to the static magnetic field (main magnetic field) for the purpose of determining arbitrarily an imaging cross-sectional plane and applying position information to RF signals from the human subject. The gradient field coils are required to provide high accuracy in the linearity of gradient fields. The shapes of the gradient field coils vary according to the direction of the static field and the directions of gradients applied to the static field. Usually, the Gz coil is in the form of a pair of loop coils and the Gx and Gy coils are each in the form of a set of four saddle-shaped coils.
The dimensions of the magnetic resonance imaging apparatus is determined by the dimensions of the static field magnet. The gantry for an apparatus of medium and high magnetic fields (0.5 to 1.5 T) is formed in the shape of a cylinder having a depth of about 2 m, an internal diameter of about 60 cm, and an outer diameter of about 2 m. The gantry is of substantially the same size in internal diameter as widths of human bodies (the average width is about 40 cm). The above noted size of the gantry is considered to be a minimum size required to produce a sufficiently uniform static magnetic field and sufficiently linear gradient magnetic fields.
A human subject under examination allowed to have access to the space (imaging area) within the cylindrical gantry has to keep still in the dark, small space during examination and therefore may have a sensation of confinement or suffocation. Although there are differences among individuals, patients who have a severe sensation of confinement cannot undergo the examination in some cases. For this reason, the demand is increasing for apparatus that allows patients to have a greater sensation of openness when placed within the gantry.
To solve this problem, one might first propose to make the lengthwise dimension of apparatus in the direction of the static magnetic field as small as possible. To this end, it is required to shorten the axial length of the static field magnet and the gradient field coils, which will result in a reduction in the size of an area where the uniformity of the static magnetic field and the linearity of the gradient magnetic fields must be ensured and consequently in a degradation in the basic performance of the static field uniformity and gradient field linearity. Even if the axial length can be shortened, the cost would have to be increased to maintain the basic performance.
Referring now to FIG. 1, there is illustrated in side view a schematic of a conventional whole-body magnetic resonance imaging apparatus except gradient amplifiers and a controller. A static field magnet (here, a superconducting magnet) 1, a gradient field coil assembly 2, and an RF coil (reception/transmission coil) 3 are placed in the order mentioned from outside toward inside within a gantry. The coil members in both the static field magnet 1 and the gradient field coil assembly 2 are arranged symmetric along the direction of the static magnetic field (hereinafter referred to as the z direction). For this reason, the center S1 of a region S, indicated by a circle, where the static field is uniform is substantially coincident with the geometrical center 1c of the static field magnet 1. The center G1 of a region G (circular region) where the gradient fields are linear is substantially coincident with the geometrical center 2c of the gradient field coil assembly 2. Of course, the region S where the static field is uniform and the region G where the gradient fields are linear are substantially coincident with each other. These regions S and G are collectively referred to as the imaging region.
Though not shown in FIG. 1, in practice the gradient field coil assembly 2 is composed of three-channel gradient field coils that produce gradient magnetic fields in the three orthogonal directions. The current distribution in the gradient field coil (Gx or Gy coil) that produces a gradient magnetic field along the axis (x or y axis) orthogonal to the z direction is shown in FIG. 2. The Gx or Gy coil consisting of four saddle-shaped coils has two coils arranged in the z-axis direction so that the intermediate portion therebetween forms the gradient field linear region G. For this reason, current return portions 6 are positioned at the axial ends of the gradient field coil assembly 2 and the gradient field coils protrude from the gradient field linear region G in the z-axis direction. Even if use is made of the gradient field coil short in axial length, therefore, patients are subject to limitations in the sensation of openness. The same holds for the case where the static field magnet is shortened.
As described above, the conventional magnetic resonance imaging apparatus has drawbacks that, due to the arrangement of the current return portions of the gradient field coils and the arrangement of coils in the static field magnet, the space where patients are placed is small and the patients"" sensation of openness is not enough.
It is an object of the present invention to provide a whole-body magnetic resonance imaging apparatus which permits a patient to have a sensation of openness when laid down in cylindrical space within the gantry by improving the arrangement of the current return portion of the gradient field coil assembly/the coil arrangement in the static field magnet.
According to a first aspect of the present invention, a magnetic resonance imaging apparatus comprises a gantry having an imaging space to which a patient can have access; a static magnetic field generating unit provided in the gantry; and a gradient magnetic field generating unit provided in the gantry, the center of the static magnet field generating unit and the center of the gradient magnetic field generating unit being displaced from each other in the direction of a static magnetic field and the gradient magnetic field generating unit having an asymmetric coil pattern in the direction of the static magnetic field.
In this apparatus, the gradient magnetic field generating unit forms a gradient magnetic field linear region at its one end in the direction of the static magnetic field and a current return portion at its other end. The gradient magnetic field generating unit and the static magnetic field generating unit are placed so that the center of the gradient magnetic field linear region, the center of the static magnetic field uniform region and the center of the static field magnet are substantially coincident with one another.
According to the first aspect of the invention, the imaging region can be positioned to the side of one end of the gradient field coil assembly and the axial length of the gradient field coil assembly can be shortened without degrading the basic performance. In addition, the bore of one end of the gantry can be increased to reduce the patient""s sensation of confinement.
A second magnetic resonance imaging apparatus of the present invention is arranged such that, in the first magnetic resonance imaging apparatus, the static magnetic field generating unit comprises a static field magnet having an asymmetric coil pattern in the direction of the static magnetic field and the gradient magnetic field generating unit and the static magnetic field generating unit are placed so that the center of the gradient magnetic field linear region and the center of the static magnetic field uniform region are substantially coincident with each other and the center of the static field magnet and the center of the static magnetic field uniform region are displaced from each other.
According to the second invention, in addition to the advantages of the first invention, the imaging region can be positioned to the side of one end of the static field magnet, further reducing the patient""s sensation of confinement.
A third magnetic resonance imaging apparatus of the present invention is arranged such that the first or second apparatus further comprises means for entering physical data of a patient, an imaging region, and an imaging sequence, means for determining the direction in which the patient is inserted into the gantry so that the patient will not undergo magnetic stimulation, or the direction in which the patient is inserted into the gantry so that the patient has a sensation of openness, and means for displaying the direction.
According to the third invention, a patient can be laid down on the couch in a direction to allow him or her to have a sensation of openness with no feeling of magnetic stimulation.
According to a fourth aspect of the present invention, a magnetic resonance imaging apparatus comprises a gantry having a static magnetic field forming unit and an imaging space to which a patient can have access, the gantry having a first bore of a first radius and a second bore of a radius larger than the first radius; and a gradient magnetic field forming unit placed between the inner wall of the gantry of the first bore and the static magnetic field forming unit.
In this apparatus, the center of the first bore is displaced from the center of the gantry in the direction of a static magnetic field. The gradient magnetic field forming unit comprises a coil having a coil pattern such that its magnetic field center is positioned nearer to the second bore side than to its center. The second bore is larger than the internal diameter of the gradient magnetic field forming unit. The static magnetic field forming unit comprises a coil having a coil pattern such that its magnetic field center is coincident with its center.
In the fourth invention as well, the imaging region can be positioned to the side of one end of the gradient field coil assembly and the axial length of the gradient field coil assembly can be shortened without impairing the basic performance. In addition, the bore of one end of the gantry can be increased to reduce the patient""s sensation of confinement.
In a fifth magnetic resonance imaging apparatus, the static magnetic field forming unit comprises a coil having a coil pattern such that its magnetic field center is displaced from its center.
According to the fifth invention, in addition to the advantages of the fourth invention, the imaging region can be positioned to the side of one end of the static field magnet, further reducing the patient""s sensation of confinement.
A sixth magnetic resonance imaging apparatus of the present invention is arranged such that the fourth apparatus further comprises means for entering physical data of a patient, an imaging region, and an imaging sequence, means for determining the direction in which the patient is inserted into the gantry so that the patient will not undergo magnetic stimulation or the direction in which the patient is inserted into the gantry so that the patient has a sensation of openness, and means for displaying the direction.
According to the fifth invention, a patient can be laid down on the couch in a direction to allow him or her to have a sensation of openness with no feeling of magnetic stimulation.
Additional objects and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention.
The objects and advantages of the present invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.